Early work relating to the field of Wireless Location has been described in U.S. Pat. Nos. 5,327,144, Jul. 5, 1994, “Cellular Telephone Location System”; and U.S. Pat. No. 5,608,410, Mar. 4, 1997, “System for Locating a Source of Bursty Transmissions.” Both patents are owned by the assignee of the present invention, TruePosition, Inc. TruePosition has conducted extensive experiments with Wireless Location System technology to demonstrate both the viability and value of the technology. For example, several experiments were conducted during several months of 1995 and 1996 in the cities of Philadelphia and Baltimore to verify the system's ability to mitigate multipath in large urban environments. Then, in 1996 the assignee constructed a system in Houston that was used to test the technology's effectiveness in that area and its ability to interface directly with E911 systems. Then, in 1997, the system was tested in a 350 square mile area in New Jersey and was used to locate real 9-1-1 calls from real people in trouble. The system test was then expanded to include 125 cell sites covering an area of over 2,000 square miles. In 1998, the assignee added digital radio capability to the WLS and fielded multiple dual-mode test systems including: a 16 site AMPS/TDMA system in Wilmington Del., a 7 site AMPS/TDMA system in Redmond Wash., a 38 site rural AMPS/TDMA system around Fort Wayne, Ind., a 19 site AMPS/CDMA system in King of Prussia Pa., a 33 site dense urban AMPS/CDMA system on Manhattan Island in New York, and a 135 site AMPS/CDMA system in New Jersey, Delaware and Pennsylvania. TruePosition is currently in the process of deploying a commercial 16,000 site nationwide AMPS/TDMA and GSM system. During all of these tests and commercial deployments, various techniques were tested for effectiveness and further developed.
The value and importance of the Wireless Location System has been acknowledged by the wireless communications industry. In June 1996, the Federal Communications Commission issued requirements for the wireless communications industry to deploy location systems for use in locating wireless 9-1-1 callers, with a deadline of October 2001. The location of wireless 9-1-1 callers will save response time, save lives, and save enormous costs because of reduced use of emergency response resources. In addition, numerous surveys and studies have concluded that various wireless applications, such as location sensitive billing, fleet management, and others, will have great commercial value in the coming years.
TruePosition has continued to develop systems and techniques to further improve the accuracy of Wireless Location Systems while significantly reducing the cost of these systems. For example, the following commonly-assigned patents have been awarded for various improvements in the field of Wireless Location:    1. U.S. Pat. No. 6,519,465 B2, Feb. 11, 2003, Modified Transmission Method for Improving Accuracy for E-911 Calls;    2. U.S. Pat. No. 6,492,944 B1, Dec. 10, 2002, Internal Calibration Method for a Receiver System of a Wireless Location System;    3. U.S. Pat. No. 6,483,460 B2, Nov. 19, 2002, Baseline Selection Method for Use in a Wireless Location System;    4. U.S. Pat. No. 6,463,290 B1, Oct. 8, 2002, Mobile-Assisted Network Based Techniques for Improving Accuracy of Wireless Location System;    5. U.S. Pat. No. 6,400,320, Jun. 4, 2002, Antenna Selection Method For A Wireless Location System;    6. U.S. Pat. No. 6,388,618, May 14, 2002, Signal Collection System For A Wireless Location System;    7. U.S. Pat. No. 6,351,235, Feb. 26, 2002, Method And System For Synchronizing Receiver Systems Of A Wireless Location System;    8. U.S. Pat. No. 6,317,081, Nov. 13, 2001, Internal Calibration Method For Receiver System Of A Wireless Location System;    9. U.S. Pat. No. 6,285,321, Sep. 4, 2001, Station Based Processing Method For A Wireless Location System;    10. U.S. Pat. No. 6,334,059, Dec. 25, 2001, Modified Transmission Method For Improving Accuracy For E-911 Calls;    11. U.S. Pat. No. 6,317,604, Nov. 13, 2001, Centralized Database System For A Wireless Location System;    12. U.S. Pat. No. 6,281,834, Aug. 28, 2001, Calibration For Wireless Location System;    13. U.S. Pat. No. 6,266,013, Jul. 24, 2001, Architecture For A Signal Collection System Of A Wireless Location System;    14. U.S. Pat. No. 6,184,829, Feb. 6, 2001, Calibration For Wireless Location System;    15. U.S. Pat. No. 6,172,644, Jan. 9, 2001, Emergency Location Method For A Wireless Location System;    16. U.S. Pat. No. 6,115,599, Sep. 5, 2000, Directed Retry Method For Use In A Wireless Location System;    17. U.S. Pat. No. 6,097,336, Aug. 1, 2000, Method For Improving The Accuracy Of A Wireless Location System;    18. U.S. Pat. No. 6,091,362, Jul. 18, 2000, Bandwidth Synthesis For Wireless Location System;    19. U.S. Pat. No. 5,608,410, Mar. 4, 1997, System For Locating A Source Of Bursty Transmissions; and    20. U.S. Pat. No. 5,327,144, Jul. 5, 1994, Cellular Telephone Location System.
Other exemplary patents in this field include:    1. U.S. Pat. No. 6,546,256, Apr. 8, 2003, Robust, Efficient, Localization System;    2. U.S. Pat. No. 6,366,241, Apr. 2, 2002, Enhanced Determination Of Position-Dependent Signal Characteristics;    3. U.S. Pat. No. 6,288,676, Sep. 11, 2001, Apparatus And Method For Single Station Communications Localization;    4. U.S. Pat. No. 6,288,675, Sep. 11, 2001, Single Station Communications Localization System;    5. U.S. Pat. No. 6,047,192, Apr. 4, 2000, Robust, Efficient, Localization System;    6. U.S. Pat. No. 6,108,555, Aug. 22, 2000, Enhanced Time Difference Localization System;    7. U.S. Pat. No. 6,101,178, Aug. 8, 2000, Pseudolite-Augmented GPS For Locating Wireless Telephones;    8. U.S. Pat. No. 6,119,013, Sep. 12, 2000, Enhanced Time-Difference Localization System;    9. U.S. Pat. No. 6,127,975, Oct. 3, 2000, Single Station Communications Localization System;    10. U.S. Pat. No. 5,959,580, Sep. 28, 1999, Communications Localization System; and    11. U.S. Pat. No. 4,728,959, Mar. 1, 1988, Direction Finding Localization System.
Over the past few years, the cellular industry has increased the number of air interface protocols available for use by wireless telephones. The industry has also increased the number of frequency bands in which wireless or mobile telephones may operate, and has expanded the number of terms that refer or relate to mobile telephones to include “personal communications services”, “wireless”, and others. The changes in terminology and increases in the number of air interface protocols do not change the basic principles and inventions discovered and enhanced by the assignee of the present invention.
As mentioned, there are numerous air interface protocols used for wireless communications systems. These protocols are used in different frequency bands, both in the U.S. and internationally. The frequency band generally does not impact the Wireless Location System's effectiveness at locating wireless telephones.
All air interface protocols use two types of “channels”. The first type includes control channels that are used for conveying information about the wireless telephone or transmitter, for initiating or terminating calls, or for transferring bursty data. For example, some types of short messaging services transfer data over the control channel. In different air interfaces, control channels are known by different terminology but the use of the control channels in each air interface is similar. Control channels generally have identifying information about the wireless telephone or transmitter contained in the transmission. The second type of channel includes voice channels, also known as traffic channels, that are typically used for conveying voice communications over the air interface. These channels are used after a call has been set up using the control channels. Voice channels will typically use dedicated resources within the wireless communications system whereas control channels will use shared resources. This distinction can make the use of control channels for wireless location purposes more cost effective than the use of voice channels, although there are some applications for which regular location on the voice channel is desired. Voice channels generally do not have identifying information about the wireless telephone or transmitter in the transmission.
Some of the differences in the air interface protocols are discussed below:
AMPS—This is the original air interface protocol used for cellular communications in the U.S. In the AMPS system, separate dedicated channels are assigned for use by control channels (RCC). According to the TIA/EIA Standard IS-553A, every control channel block must begin at cellular channel 313 or 334, but the block may be of variable length. In the U.S., by convention, the AMPS control channel block is 21 channels wide, but the use of a 26-channel block is also known. A reverse voice channel (RVC) may occupy any channel that is not assigned to a control channel. The control channel modulation is FSK (frequency shift keying), while the voice channels are modulated using FM (frequency modulation).
N-AMPS—This air interface is an expansion of the AMPS air interface protocol, and is defined in EIA/TIA standard IS-88. The control channels are substantially the same as for AMPS, but the voice channels are different. The voice channels occupy less than 10 KHz of bandwidth, versus the 30 KHz used for AMPS, and the modulation is FM.
TDMA—This interface is also known D-AMPS, and is defined in EIA/TIA standard IS-136. This air interface is characterized by the use of both frequency and time separation. Control channels are known as Digital Control Channels (DCCH) and are transmitted in bursts in timeslots assigned for use by DCCH. Unlike AMPS, DCCH may be assigned anywhere in the frequency band, although there are generally some frequency assignments that are more attractive than others based upon the use of probability blocks. Voice channels are known as Digital Traffic Channels (DTC). DCCH and DTC may occupy the same frequency assignments, but not the same timeslot assignment in a given frequency assignment. DCCH and DTC use the same modulation scheme, known as π/4 DQPSK (differential quadrature phase shift keying). In the cellular band, a carrier may use both the AMPS and TDMA protocols, as long as the frequency assignments for each protocol are kept separated.
CDMA—This air interface is defined by EIA/TIA standard IS-95A. This air interface is characterized by the use of both frequency and code separation. However, because adjacent cell sites may use the same frequency sets, CDMA is also characterized by very careful power control. This careful power control leads to a situation known to those skilled in the art as the near-far problem, which makes wireless location difficult for most approaches to function properly (but see U.S. Pat. No. 6,047,192, Apr. 4, 2000, Robust, Efficient, Localization System, for a solution to this problem). Control channels are known as Access Channels, and voice channels are known as Traffic Channels. Access and Traffic Channels may share the same frequency band but are separated by code. Access and Traffic Channels use the same modulation scheme, known as OQPSK.
GSM—This air interface is defined by the international standard Global System for Mobile Communications. Like TDMA, GSM is characterized by the use of both frequency and time separation. The channel bandwidth is 200 KHz, which is wider than the 30 KHz used for TDMA. Control channels are known as Standalone Dedicated Control Channels (SDCCH), and are transmitted in bursts in timeslots assigned for use by SDCCH. SDCCH may be assigned anywhere in the frequency band. Voice channels are known as Traffic Channels (TCH). SDCCH and TCH may occupy the same frequency assignments but not the same timeslot assignment in a given frequency assignment. SDCCH and TCH use the same modulation scheme, known as GMSK. The GSM General Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE) systems reuse the GSM channel structure, but can use multiple modulation schemes and data compression to provide higher data throughput.
Within this specification, a reference to control channels or voice channels shall refer to all types of control or voice channels, whatever the preferred terminology for a particular air interface. Moreover, there are many more types of air interfaces (e.g., IS-95 CDMA, CDMA 2000, and UMTS WCDMA) used throughout the world, and, unless the contrary is indicated, there is no intent to exclude any air interface from the inventive concepts described within this specification. Indeed, those skilled in the art will recognize other interfaces used elsewhere are derivatives of or similar in class to those described above.
The preferred embodiments of the inventions disclosed herein have many advantages over other techniques for locating wireless telephones. For example, some of these other techniques involve adding GPS functionality to telephones, which requires that significant changes be made to the telephones. The preferred embodiments disclosed herein do not require any changes to wireless telephones, and so they can be used in connection with the current installed base of over 65 million wireless telephones in the U.S. and 250 million wireless telephones worldwide.